The only explanations I see (for low AMD RAC compared to your Pentium) are:
- you don't let your AMD Computer to run all the time (the Computer sleeps after a period of inactivity (and not only the monitor); or you use it only 1-2 hours per day and then shutdown it)
- you have local setting in BOINC on your AMD Computer to stop computations after a period of inactivity (no mouse move/keyboard press)
- you have setting in BOINC to not let it compute when user is active but you (your family) uses this computer almost all the time when it is ON.

(on your AMD Computer you have only:
Tasks 'In progress' 8
but
Average turnaround time 12.56 days
which means this computer 'makes' < one task per day
(it can do ~10 'normal' or ~40 'short' tasks per day using the same SETI stock app for CPU that you are using now)
)

Click on 'Details' at left and Copy/Paste here this info for both computers:

% of time BOINC is running 98.1398 %
While BOINC running, % of time host has an Internet connection 99.9983 %
While BOINC running, % of time work is allowed 99.8557 %
Task duration correction factor 1.051192

It is obvious that your AMD is computing at normal (good) speed (when it is computing)
but it is computing only on small part of the day (maybe only for 1 hour)